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In the Middle Ages, representatives of the villages of Biscay would hold assemblies under local big trees. As time passed, the role of separate assemblies was superseded by the Guernica Assembly in 1512, and its oak would acquire a symbolic meaning, with actual assemblies being held in a purpose-built hermitage-house (the current building is from 1833). It was the Spanish regent Maria Christina accompanied by her infant daughter Queen Isabella II the last Spanish monarch to swear an oath to the charters under the iconic oak in 1839.

the "old tree" (1742–1892), re-planted in 1811. The trunk now is held in a templet in the surrounding garden.

the third (1858–2004), re-planted in 1860, survived the Bombing of Guernica in 1937 but had to be replaced because of a fungus. The gardeners of the Biscayan government keep several spare trees grown from the tree's acorns.

the fourth (1986-2015) was replanted on the site of its father on 25 February 2005. It died of a humidity related disease on 15 January 2015.

the fifth was planted in March 2015, aged 14.

The tree's significance is illustrated by an event which occurred shortly after the Guernica bombings. When the Francoist troops took the town, the Tercio of Begoña, formed by Carlist volunteers from Biscay, put an armed guard around the tree to protect it against the Falangists, who had wanted to fell this symbol of Basque nationalism.[2]

An oak tree is depicted on the heraldic arms of Biscay and subsequently on the arms of many of the towns of Biscay. An oak leaf logo is being used by the local government of Biscay. The logo of the Basque nationalist party Eusko Alkartasuna has one half red and the other green, the colors of the Basque flag. An old version of the logo of the nationalist youth organisation Jarrai also display oak leaves.

The Basque authorities present descendants of the tree as a symbol of friendship to Basque diaspora groups and related cities.